With the emergence of fetal hemoglobin (HbF)-stimulating agents as potential treatments for sickle-cell disease, procedures to monitor the effect of these agents on HbF levels in patients are needed. Flow cytometric assay was developed as a rapid procedure to detect fetal hemoglobin in erythrocytes (F cells) and reticulocytes (F reticulocytes) using a Tricolor-conjugated monoclonal antibody against HbF. In order to detect fetal hemoglobin in reticulocytes, these same erythrocytes are subsequently incubated with a staining solution of Thiazole Orange. This method permits rapid distinction and semi-quantitation of F cells and F reticulocytes in EDTA anticoagulated peripheral blood. In collaboration with Dr. Mark Gladwin group, we are following patients with sickle syndromes that are treated with hydroxyurea (HU) and/or erythropoietin or darbepoietin (both referred to as EPO). Flow cytometric analysis is used to establish baseline levels of HbF and F reticulocytes in SCD patients and to assess their responses to different treatment regimens. The percent of HbF and F-cells increase in all evaluable patients is recorded and HU dosing in conjunction with EPO therapy is explored, particularly in high-risk SCD patients and in the setting of renal insufficiency, common to the aging sickle cell population.